The present invention relates to an apparatus for removing acidic gases, such as sulfur dioxide and hydrogen chloride gases, from a hot gas, such as the flue gas produced during the combustion of a sulfur bearing fossil fuel or refuse. More particularly, it relates to a spray drying apparatus and process for scrubbing hot gases wherein the gas to be scrubbed is first contacted with a spray of absorbent liquid and then passes through a foraminous collecting member upon which is deposited particulate material originally entrained in the gas, as well as particulat salts formed by reaction of the absorbent with the acidic gases.
Sulfur oxides are produced during the combustion of sulfur bearing fossil fuels, such as oil and coal. Other acidic gases, such as hydrogen chloride, are produced in addition to sulfur oxides during the combustion of refuse such as household garbage and industrial waste. The adverse effect on the environment of discharging such acidic gases into the atmosphere is well established and has led to legislation greatly restricting the amount of sulfur oxide and other gaseous pollutants which may be emitted to the atmosphere.
One particular spray dryer apparatus and process well suited for use in scrubbing acidic gas from a hot flue gas is disclosed in U.S. Pat. No. 4,504,451. As disclosed therein, the hot flue gas to be scrubbed is passed to a primary drying chamber and contacted therein with a spray of droplets of an absorbent liquid containing particles of an alkali or alkaline reactant. The droplets are partially dried in the primary drying chamber to a tacky condition before reaching a foraminous collecting member extending through a lower portion of the primary drying chamber whereon it collects as a moist, porous mat. As the flue gas continues to flow through the mat depositing on the collecting member, the mat serves as a filter for removing particulates, including fly ash, in the gas and the remaining sulfur oxides in the gas react with unreacted particles of reactant and reactive ingredients of the fly ash collected in the mat. The flue gas leaving the primary drying chamber is passed to a secondary drying chamber wherein it is again passed through the mat which is translated across the secondary drying chamber from the primary drying chamber. As the gas passes through the mat in the secondary chamber, water is evaporated from the mat to dry the particulate material forming the mat thereby facilitating its subsequent removal.
In the operation of such a scrubbing apparatus, the flue gas leaving the secondary drying chamber is typically clean enough during normal steady state operation to be vented directly to the atmosphere. However, during initial start-up when the foraminous member is clean having no particulate material build-up, and in the latter stages of start-up before sufficient particulate has been deposited on the foraminous collecting member to form an effective tacky mat for removing particulate material from the hot gas, the gas leaving the secondary drying chamber may not be clean enough to vent directly to the atmosphere depending upon existing particulate emission regulations. Therefore, it has been customary to make provision for back-up particulate collection by passing the gas leaving the secondary drying chamber through an additional downstream particulate collector, such as a cyclone, a fabric filter or an electrostatic precipitator, prior to venting the flue gas to the atmosphere. Unfortunately, the provision of an additional downstream particulate collector, which is generally not necessary at steady-state operation, results in an undesirable increase in overall system pressure drop experienced by the gas in traversing the system thereby adversely increasing fan power consumption.
Accordingly, it is an object of the present invention to provide a scrubbing apparatus of the mat type wherein a supplementary particulate filter means is provided during start-up operation but removed from the gas flow path during steady-state operation.